1. Field of the Invention
The invention relates to a bandpass filter that follows the frequency of an input signal especially for reducing the noise on the receive band of mobile communications devices.
2. Description of the Related Art
In two-way radio apparatus part of the noise on the receive band is generated by the transmission circuitry in the apparatus. To reduce such noise in GSM mobile stations, for example, a transmitter specification specifies that the noise power density on the receive band shall not exceed xe2x88x9279 dBm/100 kHz. Taking into account the signal-to-noise ratio and maximum transmission power of commercial quadrature modulators, an additional attenuation of at least 12 dB is needed on the receive band in the transmission circuit.
From the prior art it is known to use a duplex filter to provide sufficient attenuation on the receive band. A disadvantage of the duplex filter is that it attenuates the transmitted signal, too. Therefore, the power of the output stage has to be increased correspondingly, which results in a considerable increase in the current consumption of the apparatus.
In addition, an arrangement is known in which the noise on the receive band is attenuated by a filter fixed in front of the output stage and by a notch filter. The stop band of the notch filter is situated at the bottom end of the transmit frequency band and it is switched on only when the upper channels are being used. Noise transmitted from the bottom end of the transmit band to the receive band because of the nonlinearity of the output stage is thus attenuated. The disadvantage of such an arrangement is its technical complexity and, consequently, relatively large extra costs.
Furthermore, an arrangement is known in which the modulation result at first falls within a relatively low constant frequency range. The noise-attenuating filtering can in that case be realized by means of one and the same low-pass filter independent of the RF channel. FIG. 1 shows such a structure according to the prior art. It comprises a modulator 11, phase difference detector 12, low-pass filter 13, which is the noise-attenuating filter mentioned above, and a voltage-controlled oscillator 14 that produces a radio-frequency signal RF OUT. A control signal n determines the RF channel. The RF signal from the oscillator 14 is multiplied in a mixer 16 with a signal from a synthesizer 15, the frequency of which depends on the channel selected, producing signal A which is always in the same frequency range. This is taken to the modulator 11 where it is quadratically modulated using baseband signals I and Q. The result is signal B the phase of which is compared with the phase of a constant-frequency sine wave. Said constant frequency fref determines the frequencies of signals A and B. If the frequency or phase of signal B changes because of modulation, the output signal C from filter 13 controls the oscillator 14 in such a manner that a corresponding change occurs in the output signal RF OUT. The change is corresponding because of the feedback via mixer 16. A disadvantage of the method is that it is not suitable for modulations in which the carrier amplitude varies, too.
An object of the invention is to provide a solution for tuning a resonator, which solution makes it possible to reduce above-mentioned disadvantages associated with the prior art.
The basic idea of the invention is as follows: An input signal, which may be e.g. a modulated carrier wave, feeds a resonator-type bandpass filter the resonance frequency of which is electrically adjustable. The resonance frequency is forced to follow the input signal frequency on the basis of the phase of the resonator voltage: If the frequency of the input signal, which equals the frequency of the resonator voltage, changes, the phase of the resonator voltage changes, too. A signal is generated which is proportional to the phase difference between the input signal voltage and resonator voltage. This signal is used to control the reactance of the resonator in such a manner that the resonance frequency changes in the same direction as the frequency of the input signal. As the resonance frequency reaches the new frequency of the input signal, said phase difference has decreased so that it is near zero and the resonance frequency stops changing. Thus the resonance frequency follows the frequency of the input signal.
An advantage of the invention is that the resonance frequency of the resonator can be set relatively accurately to the input signal frequency because the phase characteristic of the resonator is at its steepest at the resonance frequency whereby the phase difference detector reacts strongly on even the slightest frequency changes. As the resonance frequency can be set accurately, the filter comprised of the resonator can be adapted so as to attenuate noise relatively close to the input signal frequency. Thus e.g. the above-mentioned noise attenuation on the receive band of the GSM network can be realized without complicated additional arrangements. Another advantage of the invention is that the structure according to the invention allows for the use of modulations resulting in carrier wave amplitude changes. A further advantage of the invention is that the tuning to the new frequency is very quick. A yet further advantage of the invention is that the manufacturing costs of the structure according to the invention are relatively low.
The method according to the invention for tuning a resonator, in which method an input signal feeds a resonator in such a manner that the resonator oscillates at the frequency of the input signal and the center frequency of the pass band of the resonator is set substantially to the input signal frequency, is characterized in that in order to set the center frequency
there is generated a difference signal proportional to the phase difference between the input signal voltage and resonator voltage, and
the resonance frequency of the resonator is changed by means of the difference signal in such a manner that said phase difference becomes smaller.
The arrangement according to the invention for tuning a resonator is characterized in that the arrangement comprises
means for generating a difference signal proportional to the phase difference of an input signal voltage and resonator voltage, and
means for changing the resonance frequency of the resonator in such a manner that said phase difference becomes smaller.
Preferred embodiments of the invention are described in the dependent claims.